The ability to disconnect and then anastomose the periosteal and endosteal blood vessels supplying a sizable piece of cortical bone by microsurgery has made possible for the first time a means of studying important aspects of cell viability in bone tissue in both autogenous and allogenic bone grafts. This affords the opportunity to study the nutrient vessels themselves and the recipient artery and veins to which they are anastomosed outside the bone graft by light microscopy; the maintenance of vascularity by arteriography and bone scintography and the bone formation process in the graft by fluorochrome (e.g. tetracycline) labeling techniques. It also gives the opportunity to study by electron microscopy the effect of transplantation on the morphology of the endothelial cells, Schwann cells and their neurofibrils, some marrow cells just peripheral to these endothelial cells and the bone cells (i.e. pre-osteo-blasts, osteoblasts, osteoclasts and osteocytes) satellite to the capillary-type vessels which are the core of the osteons inside those bone grafts. Changes in cell structure can be more readily detected by EM than by light microscopy. Electron microscopy combined with histochemistry should permit the study of such factors as the effect of various elapsed times without circulation on the cells in osteons (Haversian systems) in the cortex of rib grafts between isolation of the graft and its vascular anastomosis in the recipient site; differences that may attend the use of various "holding solutions" during various periods of elapsed times before anastomosis of the recipient vein and aetery; the relative viability of osteoblasts and osteocytes by changes in their morphology and histochemistry after various periods in situ, with and without circulation and in the case of the allogenic bone grafts, with and without immunosuppression. Vascularized cortical bone grafts are clinically important in replacing large segments of long bones. This study should increase our knowledge of cellular events in such grafts and let us increase their viability. Also it will assist us to understand changes in future studies on vascularized epiphyseal plate transfers.